Referring to FIG. 1, there is shown a schematic diagram of two photodiodes sharing an amplifier, the source follower transistor SF. The photodiodes transfer their charge to their respective floating diffusions FD1 and FD2 via their respective transfer gates TG1 and TG2.
Referring to FIG. 2, there is shown a schematic of a typical prior art image sensor having four photodiodes PD1, PD2, PD3 and PD4 shared by an amplifier, the source follower SF. Similar to FIG. 1, the operation of FIG. 2 includes the photodiodes PD1, PD2, PD3 and PD4 transferring their charge to their respective floating diffusions FD1, FD2, FD3 and FD4 via their respective transfer gates TG1, TG2, TG3 and TG4.
Referring to FIGS. 3 and 4, there are respectively shown top views of FIGS. 1 and 2. The image sensor includes the plurality of photodiodes and their associated Field Effect Transistors (FETs) placed asymmetrically within the image sensor unit cell and at various distances from the photodiodes. The FETs function to provide various functions for the image sensor such as amplification, reset and row selection. The design of FIGS. 1 and 2 maximizes the optical fill factor, but it can also have optical gain fixed pattern noise due to optical and electrostatic asymmetry of the regions surrounding each photodiode.
Although the prior art image sensor is satisfactory, it includes drawbacks such as optical gain fixed pattern noise. Therefore, a need exists for reducing this optical gain fixed noise pattern while maximizing optical fill factor.